The present invention relates to field devices. More particularly, the present invention relates to process interfaces between the field device and the process.
Field devices, such as process variable transmitters, are used by a number of industries to remotely sense a process variable. Such variables are generally associated with fluids such as slurries, liquids, vapors, gases, chemicals, pulp, petroleum, pharmaceuticals, food, and other fluid processing plants. Process variables may include pressure, temperature, flow, turbidity, density, concentration, chemical compensation, and other properties. Other examples of field devices include valves, actuators, heaters, and controllers.
Process variable transmitters are used to measure and provide accurate and reliable process measurements. One of the challenges in making accurate and reliable process measurements is maintaining the integrity of the process interface and the process medium itself. It is common for the process fluid to clog or solidify due to changing temperatures or changes in the state of the fluid itself leading to erroneous measurements and potentially unsafe process conditions.
The use of thermal control systems in industrial process control and measurement is known. For example, high purity vacuum transducers often have an internal thermal control system to maintain the entire device at a selected temperature in order to increase accuracy and/or longevity of the device. Additionally, some field devices employ a thermal control system disposed proximate a primary element in order to ensure that the element is maintained at a desired temperature. For example, it is known for pitot tubes to be heated such that they do not accumulate ice in measuring air velocity during flight.
Additionally, some have employed a number of external means and methods to apply thermal control systems to field devices. These techniques generally employ electrical heat elements or steam tracing but are difficult to install, have poor temperature measurement and control, and are costly and troublesome to maintain. These devices are “add-on designs” designs that are typically attached externally to the connection hardware or measurement instrument itself. While prior approaches have generally addressed some thermal issues of the instrument modules and primary elements themselves, the process interface element has not been utilized for such uses. Installations with thermal control elements added-on to the process interface element require additional control systems, additional installation time and expense. Further, such systems are more susceptible to failure since they are exposed to the elements. Thus, there is a need for field devices having process interface elements with more integral thermal control systems. Such field devices would provide the advantages of thermal control of the process interface less expensively and more robustly.